1. Field of the Invention
The present invention relates to a polishing apparatus. More particularly, the invention relates to a polishing apparatus that is suitable for use in the chemical mechanical polishing (CMP) process performed in manufacturing semiconductor devices.
2. Description of the Related Art
In recent years, the integration density of semiconductor devices has increased, making it possible to form a multilayer interconnection structure in each semiconductor device. To manufacture a semiconductor device having the multilayer interconnection structure, each layer of the multilayer interconnections should have so flat a surface that projections and depressions, if any, may fall within the focal depth of the light source used in photolithography. The CMP process is employed to impart such a flat surface to the each film.
FIG. 5 shows a polishing apparatus 100 designed for use in the CMP process. The polishing apparatus 100 includes a polishing pad 11 that has a circular polishing surface 11a. On the polishing surface 11a, a wafer 12 is mounted, and a polishing head 13 is positioned thereon to sandwich the wafer 12 between the same and the polishing surface 11a. The polishing head 13 includes a head body 14, a retainer ring 16, and a membrane sheet 17. The head body 14 is of a disc shape The membrane sheet 17 has a diameter substantially equal to the diameter of the wafer 12, contacts the rear surface of the wafer 12, and is disposed below the head body 14. The retainer ring 16 is disposed to surround the wafer 12 and the membrane sheet 17. The bottom of the head body 14 and the membrane sheet 17 define a closed space 18. High-pressure air is supplied into the closed space 18, whereby the wafer 12 is depressed toward the polishing pad 11.
The retainer ring 16 has a top surface secured to the bottom of the head body 14 and a bottom surface in contact with the polishing surface 11a of the polishing pad 11. FIG. 6 is a top plan view illustrating the positional relationship between the wafer 12 and the retainer ring 16. The retainer ring 16 contacts, at the inner periphery thereof, the periphery of the wafer 12 to retain the wafer 12 in a horizontal direction, i.e., in-plane direction of the wafer 12.
During polishing the wafer 12, the polishing pad 11 is rotated around the axis thereof, while slurry, i.e., polishing agent, is being supplied onto the center of the polishing surface 11a. The membrane sheet 17 depresses the rear surface of the wafer 12 retained within the retainer ring 16. The main surface of the wafer 12 is thereby depressed onto the polishing surface 11a. In this condition, the polishing head 13 is rotated around the axis thereof and moved back and forth in the radial direction of the polishing surface 11a, whereby the polishing head 13 polishes the main surface of the wafer 12 in the CMP process.
JP 2004-119495A and JP 2002-367941A, for example, describe apparatus similar to the polishing apparatus of FIG. 5 and a polishing method such as described above.
In the conventional polishing apparatus 100, the periphery of the membrane sheet 17 is aligned with the periphery of the wafer 12 as shown in FIG. 5. However, the membrane sheet 17 is deformed to bulge downwards, when high-pressure air is supplied into the closed space 18 to conduct the polishing. As a result, the periphery of the membrane sheet 17 is raised as is indicated by numeral 31 in FIG. 7. Inevitably, the pressure depressing the wafer 12 downwards will be decreased in the vicinity of the periphery of the wafer 12.
The decrease in the pressure depressing the wafer 12 downwards reduces the polishing rate during the CMP process to cause a non-uniform surface, i.e., a lager range of variation on the polished surface. This ultimately lowers the product yield in the manufacture of semiconductor devices. To solve this problem, the assigner of the present invention has a proposal as described in JP Patent Application No. 2006-136024 that the membrane sheet 17 should have a larger diameter than the wafer 12 so that the periphery of the membrane sheet 17 may not be raised and may not be brought into contact with the wafer 12 during the polishing process.